In a CDMA system, such as a cdma2000/IS2000 system or Wideband CDMA (WCDMA) system, a searcher element is used in the mobile station receiver demodulator for acquisition, set maintenance and control of the demodulating element, also referred to as finger assignment. In the Idle State or in the Traffic State, the searcher derives multi-path amplitude profiles corresponding to the Active, Candidate and Neighbor base stations. A mobile station controller uses the derived multi-path amplitude profiles to assign demodulator fingers in order to demodulate and combine the signals received from the strongest ones of the multi-paths.
In accordance with conventional operation the searcher derives multi-path profiles by correlating the incoming signal with a replica of the PN sequence over a number of PN time offset hypotheses. Typically, the searcher produces a two sample/chip multi-path profile. This profile corresponds to the amplitude of the complex correlation of the incoming data with the PN replica over a certain correlation length at two samples/chip.
It should be noted that the reference to amplitude implies a magnitude calculation. The multi-path profile is typically implemented as an amplitude profile because the bit width of the information to transfer from the searcher hardware to the DSP/general processor is reduced. Note that instead of the amplitude or magnitude of the complex correlation, one could use the power or magnitude squared of the complex correlation, as shown in the prior art searcher embodiment of FIG. 2.
In general, the multi-path profile is the result of a convolution of the base station signal with a radio channel, the base station transmit filter and the mobile station receive filter. That is, a conventional searcher produces the multi-path profile using a convolution of the transmit/receive filters and the radio channel. The Active Set profile generated by the searcher is then used by the controller to assign the fingers to demodulate the incoming CDMA signal.
However, if the multi-path profile data is compromised, such as by the presence of excessive noise or artifacts (such as sidelobe peaks) in the profile data, there exists a potential that the controller will assign fingers incorrectly to some PN offset, whereby the overall demodulation performance of the receiver is degraded. Further, when artifacts are present they are also processed by the finger assignment algorithm, thereby increasing the overall complexity without deriving any additional benefit.